Toroidal magnetic device

ABSTRACT

A toroidal magnetic device comprising inner and outer concentric rings of compatible magnetic material having interfacing peripheral surfaces diffusion bonded to one another to form a unitary integral structure which is magnetized in the radial direction.

United States Patent [191 Reid et al. Aug. 5, 1975 TOROIDAL MAGNETIC DEVICE 3,540,945 11/1970 smm et al. 335/302 3,706,059 12/1972 Theyse 335/302 [75] Inventors: W'mam Rel, Qumcy; Albert 3,784,945 1/1974 Baermann 335/302 Gale, Sudbury, both of Mass.

[73] Assignee: Raytheon Company, Lexingto Primary E \-aminer-Harold Broome MASS Attorney, Agent, or FirmJohn T. Meaney; Harold A. [22] Filed: May 9, 1974 Murphy; Joseph D. Pannone [2]] Appl. No.: 468,606

[57] ABSTRACT [52] US. Cl. 335/302; 335/296; 335/306 [51] Int. Cl. HOlF 7/02 A toroidal magnetic device Comprising inner and 58 Field of Search 335/302, 296, 303, 304, Outer Concentric rings of Compatible magnetic mate- 335 30 rial having interfacing peripheral surfaces diffusion bonded to one another to form a unitary integral [56] References Cited structure which is magnetized in the radial direction.

N T STATE P TENTS U I ED S A 7 Claims, l6 Drawing Figures 3,233,950 2/1966 Baermann 308/10 PATENTEDAUE 5|975 3 898,599

CO I

SHEET TOROIDAL MAGNETIC DEVICE BACKGROUND OF THE INVENTION This invention relates generally to permanent magnets and is concerned more specifically with a toroidal magnetic device which is magnetized in the radial direction.

Powder metallurgical techniques provide means for manufacturing toroidal permanent magnets very efficiently from sintered powder material. As a result, these toroidal magnets exhibit extremely high energy product and temperature stability. However, the inherent mechanical weakness of the structural material presents serious problems in the handling and finishing of the permanent magnets. 1

Therefore, it is advantageous and desirable to provide a toroidal permanent magnet made of sintered powder material with a structural support member which cooperates with the permanent magnet to form a very efficient magnetic device.

SUMMARY OF THE INVENTION Accordingly, this invention provides a toroidal magnetic device comprised of inner and outer concentric rings of compatible magnetic material having interfacing peripheral surfaces diffusion bonded to one another.

One of the rings comprises a support annulus made of paramagnetic material having a relatively low magnetic retentivity and which is conformingly shaped to interfit with the other ring. The other ring comprises a radially polarized permanent magnet which is made from sintered powder material. The powder material is compacted onto a peripheral surface of the support ring while exposed to a particle-orienting magnetic field. After degaussing, the interlocked rings are subjected to the sintering temperature of the powder material whereby the compacted powder material increases in density, and also is diffusion bonded to the adjacent surface of the support ring. The resulting integral structure is placed in a suitable magnetic field for magnetizing the permanent magnet in the radial direction such that the flux lines of the radially polarized field pass through the support ring. Thus, the support ring not only provides structural support but also provides a low reluctance return path for the flux lines emanating from the permanent magnet ring.

In one embodiment of this invention, the toroidal magnetic device is provided with an inner support ring and outer permanent magnet ring.

In an alternative embodiment of this invention, the toroidal magnetic device is provided with an inner permanent magnet ring and an outer support ring.

The permanent magnet ring may be magnetized to have a circumferential magnetic pole adjacent a periphery'of the device and a continuous opposite magnetic pole adjacent the support ring.

Alternatively, the permanent magnet ring may be magnetized to provide adjacent a periphery of the device a circular array of alternate north and south magnetic poles, each of which is magnetically associated with a respective opposite magnetic pole radially located therefrom and adjacent the support ring.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention, the following more detailed description makes reference to the accompanying drawings wherein:

FIG. 1 is a plan view of one embodiment of this invention;

FIG. 2 is a fragmentary axial view of an apparatus employed for fabricating the embodiment shown in FIG. 1;

FIG. 3 is a plan view of another embodiment of this invention;

FIG. 4 is a fragmentary axial view of an apparatus employed for fabricating the embodiment shown in FIG. 3;

FIG. 5 is an axial view of an apparatus'utilized for magnetizing the embodiments shown in FIGS. 1 and 3;

FIG. 6 is a schematic elevational view of the apparatus shown in FIG. 5;

FIG. 7 is a plan view of the embodiment shown in FIG. 1 magnetized in a radially inward direction;

FIG. 8 is a plan view of the embodiment shown in FIG. 1 magnetized in a radially outward direction;

FIG. 9 is a plan view of the embodiment shown in FIG. 3 magnetized in a radially inward direction;

FIG. 10 is a plan view of the embodiment shown in FIG. 3 magnetized in a radially outward direction;

FIG. 11 is a plan view of another apparatus utilized for magnetizing the embodiments shown in FIGS. I and 3;

FIG. 12 is a schematic elevational view of the apparatus shown in FIG. 11;

FIG. 13 is a plan view of the embodiment shown in FIG. 1 magnetized as a multi-polar magnetic device;

FIG. 14 is a plan view of the embodiment shown in FIG. 13 but having the magnetic poles slightly rotated;

FIG. 15 is a plan view of the embodiment shown in FIG. 3 magnetized as a multi-polar magnetic device; and

FIG. 16 is a plan view of the embodiment shown in FIG. 15 but having the magnetic poles slightly rotated.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly to the drawings wherein like characters of reference designate like parts throughout the several views, there is shown in FIG. 1 a toroidal magnetic device 10 including inner and outer concentric rings, 12 and 14, respectively, which encircle a central aperture 16.

The outer ring 14 comprises a radially polarized permanent magnet made of sintered powder material, such as samarium cobalt, for example. Alternatively, the rare earth component of the compound may comprise one or more members of the group including lanthanum, cerium, samarium, praseodium, ytrrium, and cerium rich mischmetal. When combined with cobalt, the members of the group form intermetallic compounds having hexagonal crystalline structures with single easy axes of magnetization. The resulting magnetic material exhibits high values of coercive force, magnetic saturation, and Curie temperature which provide a'permanent magnet having extremely high energy product and temperature stability.

The inner ring 12 comprises a support annulus made of a soft magnetic material having relatively low coercive force such as cobalt, for example, which is compatible with the samarium cobalt material of permanent magnet ring 14. The conforming configuration of support ring-12 interfits with the permanent-magnet ring 14 such'thatthe outer peripheral surface of support ring 12 interfaces with the inner peripheral surface of permanent magnet ring -1 4. As a result, the magnetic permeable material of support ring 12 provides a low reluctance path for the flux of radially polarized magnet 14 thereby enhancing the efficiency of magnetic device 10. Since the material of support ring 12 is compatible with the material of magnet ring 14, the possibility of deleterious reactions occurring between the interfacing materials is minimized. Also, by having the material of support ring 12 compatible with the material of magnet ring 14, the coefficients of thermal expansion of the respective rings 12 and 14 are very close to one another, thus reducingthe possibility of thermal stresses causingrupturing or cracking of the magnetic material, as during sintering and subsequent cooling for example.

The magnetic device may be fabricated very efficiently by means of a high speed die pressing and magnetization technique to be described. A mixture comprising one-third by weight samarium and two-thirds by weight cobalt is placed in a crucible and exposed to a suitable temperature, such as 1350 C, for example, for melting the constituent elements in a protective atmosphere of inert gas, such as helium, for example. After cooling, the resulting samarium cobalt compound is pulverized, by conventional means, to yield a fine powder having particles approximately twelve microns in size, for example. .As shown in FIG. 2, a predetermined quantity of the samarium cobalt powder is circularly disposed adjacent the outer periphery of a preformed cobalt ring l2.The ring 12 is disposed in a fixture (not shown) which is positioned in a cylindrical die 22 of a suitable press 20, such as Model HPM-IOO made by Bussman Simetag of Munich, Germany, for example.

The press 20 may include a lower ram 24 which is encircled by a coil 32 of a particle-orienting electromagnet 30. Extending axially through the lower ram 24 and through the support ring 12 is a centrally located pole piece 34 of the electromagnet 30. The pole piece 34 is magnetically coupled to a cylindrical metal wall 36 having a flanged rim 38 which constitutes the other pole piece of the electromagnet 30. The samarium cobalt powder is supported in alignment with a cylindrical punch 26 of an upper ram 28 which is reciprocally movable toward and away from the lower ram 24 within the die 22.

In operation, the coil 32 of electromagnet 30 is energized to establish between the central pole piece 34 and the surrounding pole piece 38, a radially extending field of magnetic flux. As a result, the fine particles of samarium cobalt are radially oriented and aligned with adjacent particles of the powder material. Then the upper punch 28 and the lower rams 24 cooperate to compact the samarium cobalt powder onto the outer periphery of support ring 12 in the form of a densely packed ring 14 of radially oriented particles. Consequently, when the pressure exerted by the press 20 is released, the fine particles of compacted samarium cobalt material remain radially oriented, even during a subsequent degaussing operation. Thus, the support ring 12 and the compacted material of ring 14 are formed into-a unitary toroidal device 10 having interfacing peripheral surfaces.

Subsequently, the coil 32 of electromagnet 30 may be energized in the reverse direction to establish a degaussing magnetic field which removes any residual magnetism from the samarium cobalt material of ring 14. The assembled device 10 is removed from press 20 and placed in a conventional sintering furnace (not shown) such as Heavy Duty Model Omega 1 made by Lindbergh, Inc., of Watertown, Wisconsin, for exam ple. In the furnace, the device 10 is subjected to a suitable sintering temperature, such as 1 C, for example, whereby the radially aligned particles of samarium cobalt become more densely packed and fusion bonded to one another. Furthermore, it has been found that,

during the sintering operation, the samarium cobalt material of ring 14 also is diffusion bonded to the coa outer concentric rings, 42 and 44, respectively, which encircle a central aperture 46. The inner ring 42 comprises a radially polarized, permanent magnet made of sintered powder material, such as samarium cobalt, for example. The outer ring 44 comprises a support annulus made of soft magnetic material having relatively low coercive force such as cobalt, for example, and which is compatible with the material of magnet ring 42. Thus, the magnet device 40 is structurally similar to the magnetic device 10 except the permanent magnet rings and the support rings of the respective devices 10 and 40 are transposed with respect to one another.

The magnetic device 40 also may be fabricated by means of the high speed die-pressing technique described in relation to the magnetic device 10. However, as shown in FIG. 4, the press 20a utilized for fabricating the magnetic device 40 differs from the press 20 shown in FIG. 2 by the location of the upper punch 26a carried on the upper ram 28a. The punch 26a is aligned with the powdered material of the inner concentric ring 42 rather than with the outer concentric ring 14 as shown in FIG. 2. Otherwise, the preparation of the samarium cobalt powder and the compacting thereof onto the inner peripheral surface of support ring 44 is substantially the same as previously described for the fabrication of magnetic device 10. Consequently, the densely packed ring 42 forms with the support ring 44 a unitary device which is subjected to a sintering temperature environment, as described in connection with the magnetic device 10. Accordingly, during the sintering operation, the radially oriented particles of the magnet ring 42 become even more densely packed and fusion bonded to one another. Also, during the sintering operation, the samarium cobalt material of the inner ring 42 is diffusion bonded to the cobalt material of outer ring 44 to form an integral structure.

After sintering and cooling, the respective devices 10 and 40 may be magnetized in the radial direction by means of suitable magnetizing apparatus, such as Impulse Magnetizer Model 8100 made by Ra'ytheon Manufacturing Company of Lexington, Massachusetts, for example. As shown in FIGS. 5 and 6, the magnetizing apparatus may comprise a magnetizer 50 having a pair of bucking pancake coils 52 and 54 respectively, disposed in spaced parallel relationship within a magnetically permeable casing 60. The coils 52 and 54 are provided with respective central apertures 56 and 58 which are axially aligned with a pair of opposed openings 61 and 62, respectively, in the casing 60. A mag netic device, such as 10, for example, may be interposed between the respective coils 52 and 54 such that the central aperture 16 of device is axially aligned with the central apertures 56 and 58 of the coils 52 and 54, respectively. A magnetically permeable core 64 then may be inserted axially through the mutually aligned apertures 56, 16 and 58, respectively, and through the aligned openings 61 and 62 of casing 60.

The coils 52'and 54, respectively, may be connected electrically to polarized terminals of a pulser 66 in a reverse manner with respect to one another. Thus, the coils 52' and 54 may be energized to establish respective toroidal magnetic fields having, within the core 64, oppositely directed flux lines which radially enhance one another in the plane of the magnetic device 10. As a result, the radially oriented and densely packed particles of ring 14 are permanently magnetized in the radial direction relative to the device 10. Consequently, the magnet ring 14 is provided with a circumferential magnetic pole adjacent the outer periphery of device 10 and a continuous opposite magnetic pole adjacent the support ring 12.

Accordingly, as shown in FIG. 7, there may be produced a magnetic device 10a including a permanently magnetized outer ring 14a having a circumferential north magnetic pole adjacent the outer periphery of device lOa and a continuous south magnetic pole adjacent the inner support ring 12a, the poles being coupled to one another externally of the device 10a by a toroidal-shaped magnetic field. By convention, the magnetic field lines of magnet ring 14a are directed radially inward of the device from the circumferential north pole toward the continuous south pole. However, the magnetic field lines enter the south pole by way of the low reluctance path provided by the support ring 120. Thus, there is provided a magnetic device 10a having magnetic field lines which do not extend substantially inward of the central aperture 16a and which provide a uniform salient magnetic field adjacent the outer periphery of the device. Consequently, the magnetic device 10a is especially adapted to function efficiently as a journal member of a magnetically suspended friction-free bearing for example.

As shown in FIG. 8, a magnetic device 10b may be produced by reversing the respective electrical connections of the coils 52 and 54 at the polarizer 66, after magnetizing a device such as 10a, for example. The device 10b includes a permanently magnetized outer ring 14b having a circumferential south magnetic pole adja cent the outer periphery of device 10b, and a continuous north magnetic pole adjacent the inner support ring 12b. Consequently, the magnetic field lines emanating from the north pole of magnet ring 14b pass through the magnetically permeable material of support ring 12b and are directed radially outward of the device 10b. The magnetic field lines extend beyond the outer periphery of the device before entering the circumferential south pole of magnet ring 14b. Thus, the device 14b is similar to the device 10a shown in FIG. 7 and functions as efficiently when used in place thereof.

The magnetizer 50 also may be employed for magnetizing as described, the magnetic device 40 shown in FIG. 3, whereby the radially oriented and densely packed particles of magnet ring 42 will be permanently magnetized in the radial direction relative to the device 40. Consequently, the inner magnet ring 42 will be provided with a circumferential magnetic pole adjacent the inner periphery of device 40, and a continuous opposite magnetic pole adjacent the outer support ring 44.

Thus, as shown in FIG. 9, there may be produced a magnetic device 40a including a permanently magnetized inner ring 42a having a circumferential north pole adjacent the inner periphery of device 40a and a continuous south pole adjacent the outer support ring 44a. Therefore, the magnetic field lines of magnet ring 42a extend radially inward of the central aperture 46a and then are directed radially outward toward the support ring 42a. The support ring 44a provides a low reluctance path for the'magnetic flux to enter the adjacent south pole of magnet ring 42a. Accordingly, thereis provided a magnetic device 40a having magnetic field lines which extend radially inward of the central aper-' ture 46a and do not extend substantially beyondthe outer periphery of device 40a. Consequently, themagnetic device 40a is especially adapted to function efficiently as the sleeve member of a magnetically suspended friction-free bearing, for example, which has a circular array of rotor pole pieces disposed adjacent the inner periphery of the stator.

As shown in FIG. 10, a magnetic device 40b may be produced by reversing the respective electrical connections of the coils 52 and 54 at the pulser 66, after magnetizing a device such as 40a, for example. The device 40b includes a permanently magnetized inner ring 42b having a circumferential south magnetic pole adjacent to inner periphery of device 40b, and a continuous north magnetic pole adjacent the outer support ring 4417. Consequently, the magnetic lines emanating from the continuous north pole of magnet ring 42b pass through the magnetically permeable material of support ring 44b and are directed radially inward of the device. The magnetic field lines extend into the central aperture 46b before entering the circumferential south pole of magnet ring 42b. Thus, the magnetic device 40 is similar to the device 400 and functions as efficiently when used in place thereof.

Alternatively, as shown in FIGS. 11 and 12, the mag netizing apparatus may comprise a magnetizer having a circular array of spaced conductors 72 which are perpendicularly disposed with respect to a magnetic device, such as 10, for example, and positioned adjacent the outer periphery of the device. The conductors 72 are connected in series with adjacent conductors 72 of the array by respective interconnecting conductors 74. Terminal leads 76 and 78, respectively, of the circular array are electrically connected to respective polarized terminals of a'pulser 80. Thus, an energizing current i flowing from the positive terminal of the pulser 80 passes sequentially through each of the conductors 72 before returning to the negative terminal of the pulser 80. Consequently, there is established around the conductors 72 respective cylindrical magnetic fields which permanently magnetize the adjacent, densely packed particles of magnet ring 14 in the radial direction relative to the device 10.

However, the current i passing in one direction through a particular conductor 72 of the array passes in the opposite direction through the adjacent conduc' tors 72 on either side thereof. Accordingly, the magnetic field established around the particular conductor 72 has field lines which curve in the opposite angular direction with respect to the magnetic field lines established around the adjacent conductors 72 of the array. Consequently, the magnetic field lines around the particular conductor 72 enhance the field lines around the adjacent conductors 72 on either side thereof to produce respective radially inwardly and outwardly directed magnetic intensity forces. As a result, the magnet ring 14 of device 10 is provided with alternating north and south poles having radially inward and outward directed field lines, respectively.

In this manner, as shown in FIG. 13, there may be produced a magnetic device 10c having an inner support ring 120 and an outer permanent magnet ring 140. Adjacent the outer periphery of device 10c, the magnet ring 140 is provided with a circular array of alternate north and south poles, each of which is magnetically associated with a respective opposite magnetic pole radially locatedrtherefrom and adjacent the support ring 120. Thus,,the inner periphery of magnet ring 140 is provided with a similar circular array of south and north poles. Consequently adjacent arcuate portions of the magnet ring 140 are magnetized in opposing radial directions, as shown by the magnetization vectors 150. Accordingly, adjacent the inner periphery of ring 14c, the magnetic field lines enter the magnetically permeable material of support ring 12c and, adjacent the outer periphery of ring 140, the magnet field lines extend outwardly beyond the outer periphery of device 100. As a result, there is provided a magnetic device 100 having magnetic field lines which do not extend substantially into the central aperture 16c, and which provide oppositely directed, salient magnetic fields adjacent the outer periphery of device 10c. A magnetic device of this type is especially adapted to function efficiently as an armature in a polyphase generator, for example.

As shown in FIG. 14, a magnetic device 10d may be produced by reversing the respective terminal leads 76 and 78 at the pulser 80 after magnetizing a device such as 100 for example. The device 10d is similar to the device 10c shown in FIG. 13 and operates as efficiently when used in place of the device 10c.

The magnetizer 70 also may be employed, as described, for magnetizing the magnetic device 40 shown in FIG. 3. Thus, as shown in FIG. 15, there may be produced a magnetic device 40c having an inner permanent magnet ring 42c and an outer support ring 440 encircling a central aperture 460. Adjacent the inner periphery of device 400, the magnet ring 42c is provided with a circular array of alternate north and south magnetic poles, each of which is magnetically associated with a respective opposite magnetic pole radially located therefrom and adjacent the support ring 440. Thus, the outer periphery of magnet ring 420 is provided with a similar circular array of south and north poles. Consequently, adjacent arcuate portions of the magnet ring 42c are magnetized in opposing radial directions, as shown by the magnetization vectors 450. Accordingly, adjacent the outer periphery of ring 420, the magnetic field lines enter the magnetically permeable material of support ring 440, and adjacent the inner periphery of ring 420, the magnetic field lines extend inwardly of the central aperture 460. As a result, there is provided a magnetic device 400 having magnetic field lines which do not extend substantially beyond the outer periphery of the device, and which provide oppositely directed, salient magnetic fields adjacent the inner periphery of device 40c. A magnetic device of the described type is especially adapted to function effi ciently as a stator in a polyphase motor, for example.

As shown in FIG. 16, a magnetic device 40d may be produced by reversing the respective terminal lead 76 and 78 at the pulser 80, after magnetizing-a device such as 40c, for example. The device 40d is similar to the device 40c previously described and operates as efficiently when used in place of device 40c.

Thus, there has been disclosed herein a toroidal magnetic device comprising inner and outer concentric rings encircling a central aperture and having interfacing peripheral surfaces diffusion bonded to one another. One of the rings constitutes a sintered powder permanent magnet having radially oriented and densely packed particles magnetized in the radial direction relative to the magnetic device. The other ring constitutes a support annulus made of magnetically permeable material which is compatible with the sintered powder material of the permanent magnet ring and which provides a low reluctance path for the magnetic field lines of the magnet ring. The permanent magnet ring may be magnetized to have a circumferential magnetic pole adjacent a periphery of the magnetic device and a continuous opposite magnetic pole adjacent the support ring. Alternatively, the permanent magnet ring may be magnetized to have adjacent a periphery of the magnetic device a circular array of alternate north and south magnetic poles, each of which is magnetically associated with an opposite magnetic pole radially located therefrom and adjacent the support ring.

From the foregoing, it will be apparent that all of the objectives of this invention have been achieved by the structures shown and described herein. It also will be apparent, however, that various changes may be made by those skilled in the art without departing from the spirit of the invention as expressed in the appended claims. It is to be understood, therefore, all matter shown and described herein is to be interpreted as illustrative and not in a limiting sense.

We claim:

l. A toroidal magnetic device comprising inner and outer concentric rings having interfaced peripheral portions diffusion bonded to one another, one of the rings being a radially polarized permanent magnet made of compacted powder material and the other ring being a support annulus made of soft magnetic material having relatively low magnetic retentivity.

2. A toroidal magnetic device comprising inner and outer concentric rings having interfaced peripheral portions diffusion bonded to one another, one of the rings being a support annulus made of soft magnetic material and the other ring being a compacted powder permanent magnet having a circumferential magnetic pole adjacent one periphery of the device and a continuous opposite magnetic pole adjacent the support annulus.

3. A toroidal magnetic device as set forth in claim 2 wherein the inner ring comprises the support annulus and the outer ring comprises the permanent magnet having a circumferential magnetic pole adjacent the outer periphery of the device and a continuous opposite magnetic pole adjacent the support annulus.

4. A toroidal magnetic device as set forth in claim 2 wherein the outer ring comprises the support annulus and the inner ring comprises the permanent magnet having a circumferential magnetic pole adjacent the inner periphery of the device and a continuous opposite magnetic pole adjacent the support annulus.

5. A toroidal magnetic device comprising inner and outer concentric rings having interfaced peripheral portions diffusion bonded to one another, one of the rings being a support annulus made of soft magnetic material and the other ring being a compacted powder permanent magnet having adjacent one periphery of the device a circular array of alternate north and south poles, each of which is magnetically associated with a respective opposite magnetic pole radially located therefrom and adjacent the support annulus.

6. A toroidal magnetic device as set forth in claim wherein the inner ring comprises the support annulus and the outer ring comprises the permanent magnet having adjacent the outer periphery of the device a circulararray of alternate north and south magnetic poles and adjacent the support annulus a similar circular array of south and noth magnetic poles.

7. A toroidal magnetic device as set forth in claim 5 wherein the outer ring comprises the support annulus and the inner ring comprises the permanent magnet having adjacent the inner periphery of the device a circular array of alternate north and south magnetic poles and adjacent the support annulus a similar circular array of alternate south and north magnetic poles. 

1. A toroidal magnetic device comprising inner and outer concentric rings having interfaced peripheral portions diffusion bonded to one another, one of the rings being a radially polarized permanent magnet made of compacted powder material and the other ring being a support annulus made of soft magnetic material having relatively low magnetic retentivity.
 2. A toroidal magnetic device comprising inner and outer concentric rings having interfaced peripheral portions diffusion bonded to one another, one of the rings being a support annulus made of soft magnetic material and the other ring being a compacted powder permanent magnet having a circumferential magnetic pole adjacent one periphery of the device and a continuous opposite magnetic pole adjacent the support annulus.
 3. A toroidal magnetic device as set forth in claim 2 wherein the inner ring comprises the support annulus and the outer ring comprises the permanent magnet having a circumferential magnetic pole adjAcent the outer periphery of the device and a continuous opposite magnetic pole adjacent the support annulus.
 4. A toroidal magnetic device as set forth in claim 2 wherein the outer ring comprises the support annulus and the inner ring comprises the permanent magnet having a circumferential magnetic pole adjacent the inner periphery of the device and a continuous opposite magnetic pole adjacent the support annulus.
 5. A toroidal magnetic device comprising inner and outer concentric rings having interfaced peripheral portions diffusion bonded to one another, one of the rings being a support annulus made of soft magnetic material and the other ring being a compacted powder permanent magnet having adjacent one periphery of the device a circular array of alternate north and south poles, each of which is magnetically associated with a respective opposite magnetic pole radially located therefrom and adjacent the support annulus.
 6. A toroidal magnetic device as set forth in claim 5 wherein the inner ring comprises the support annulus and the outer ring comprises the permanent magnet having adjacent the outer periphery of the device a circular array of alternate north and south magnetic poles and adjacent the support annulus a similar circular array of south and noth magnetic poles.
 7. A toroidal magnetic device as set forth in claim 5 wherein the outer ring comprises the support annulus and the inner ring comprises the permanent magnet having adjacent the inner periphery of the device a circular array of alternate north and south magnetic poles and adjacent the support annulus a similar circular array of alternate south and north magnetic poles. 